In present-day data processing systesm, memories are more and more frequently being made of magnetic discs because discs have large storage capacity and require a relatively short time for magnetic read/write heads to access information situated anywhere on the discs from the moment when the heads receive an instruction to access this information from the processing system.
It is known that magnetic discs carry information in coded form on concentric circular recording tracks having a width not exceeding a few hundredths of a millimeter and formed on both faces of the discs. The tracks are identified by alloting them a serial number j (j being a whole number) between zero and (N-1), where N is a total number of recording tracks. The coded expression of a serial number j of a track is called an address. The code expressions are most frequently in bindary codes.
It is known that for small storage capacity memories, containing only one or two discs, the addresses of the tracks on one face of a disc are recorded on said face so that a maximum amount of space is received for recording data and a minimum amount of space is set aside for recording the addresses and for recording information required to servo-control the position of the magnetic head to the several tracks of the one face.
In current practice, the information is preferably distributed over equal and adjacent sectors of a circle, each sector consisting of two parts having unequal areas. The larger part contains data intended to be processed by the data-processing system to which the disc memory belongs, while the smaller part, termed a reference zone, contains the track addresses and information for identifying the tracks, i.e., the information for servo-controlling position. Generally, one face of a disc is divided into several tens of sectors.
Each reference zone contains a certain number of cells, each of which contains an item of information required to servo-control disc position to one of the tracks and one address bit, as described in United States patent application Ser. No. 765,058 entitled "Method of Writing Addresses on a Magnetic Recording Medium" filed Feb. 2, 1977, and commonly assigned with the present application.
It is to be recalled that a binary "bit" refers to binary 1 or 0 digits that are expressed either in the form of a magnetic flux or in the form of an analogue or logic electrical signal; a logic signal is able to assume only two values termed "logic zero" and "logic one."
One known magnetic recording method involves recording consecutive items of information on a magnetic medium, so there is created on each track of the medium consecutive small magnetic domains termed "elementary magnetic areas." The areas are distributed along the entire length of an annular track so that the induced magnetism in adjacent areas is of opposing value and sense; the induced magnetism in each area is oriented in a direction parallel to the surface of the disc. Each cell as defined above represents two changes in the magnetization polarity. Each of these changes may occupy one of two predetermined positions within the cell to form a binary bit.
The present trend in the development of magnetic discs is to increase the density of the information. For this purpose, increases are made both in the radial density (which means increasing the number of recording tracks per unit length along the disc radius) and in the longitudinal density (which means increasing the number of bits per unit length along the circumference of the recording tracks). It is clear that as the density of the information increases, there is a greater probability of having recording faults or errors on the disc. Because of this, there is a possibility of the sectors, as defined above, containing faults. The current practice is to provide, on each face, a number of additional sectors, termed "standby" sectors, intended to receive the information normally contained in the sectors having faults.